Adetabular prostheses, comprising at least an outer shell and an inner bearing, are commonly used in total hip replacement surgery. Ceramic bearings are often preferred to the Ultra High Molecular Weight Polyethylene alternatives for their superior wear characteristics in normal use. However, ceramic bearings, due to their brittle nature, are particularly sensitive to excessive loading on the rim of the bearing. In its mildest form, rim loading appears as a distinctive stripe of wear on the femoral head and acetabular bearing, known as “stripe wear”. At its most severe, rim loading can lead to failure of the acetabular bearing. Rim loading is caused as the femoral head briefly separates from the acetabular shell and then contacts the rim of the bearing as it reseats into the shell. This process is known as microseparation, and the degree of separation is closely related to the orientation of the acetabular shell. The more closely the open face of the shell is aligned to a vertical axis, the greater the microseparation, and hence the greater the rim damage.
The success of ceramic bearings is thus heavily dependent on operative technique and the anatomy of the patient concerned. While the recommendation is to implant the prosthesis at a reasonably closed or reduced angle in order to limit microseparation, this is not possible for all patients. If a patient's natural acetabulum is close to vertical, the surgeon is forced to open the face of the shell in order to obtain sufficient bone contact.
Microseparation in the case of UHMWPE bearings can be compensated for by extending the bearing past the rim of the supporting shell in order to provide a larger bearing surface within which the femoral head can articulate. This option is not, however, available for ceramic bearings, due to the danger of loading an unsupported area of ceramic.